This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Moloney Murine Leukemia Virus (MLV) is a simple retrovirus that has received considerable attention as a model for virus assembly, and is currently the most widely used vector in human gene therapy trials. Like most retroviruses, MLV packages two copies of its genome, proper packaging of which is critical for viral lifespan and infectivity. A region of about 75 nucleotides, which is believed to act as a 'core encapsidation signal,'has been identified within the MLV genome. This region forms a double hairpin motif, where two sets of two stem loop structures are capable of forming a 46kDa dimer via 'kissing interactions'at the tips of the loops. We propose to use cry-EM to determine the structure of ?core encapsidation signal? which will provide the precise mechanisms of genome dimerization and packaging.